1. Field of the Invention
This invention relates to a coating metal for preventing the crevice corrosion of austenitic stainless steel and a method of preventing crevice corrosion using such metal. More particularly, the invention relates to a coating metal for preventing crevice corrosion that attacks the interface of austenitic stainless steel and another object both of which are in a liquid. The invention also relates to a method of preventing such crevice corrosion.
2. Description of the Prior Art
Apparatus, equipment and component parts which are kept in contact with seawater or other corrosive liquids are made of a corrosion-resistant metallic material which is selected from among cast iron, copper alloy and stainless steel and other materials depending upon the hostility of the environments in which such material is used. Among these corrosion-resistant materials, austenitic stainless steel is known to be particularly effective and has been employed in a wide range of corrosive environments. Austenitic stainless steel exhibits the desired effect in an environment where the corrosive liquid is moving, but as the flow rate of the fluid decreases, and diffusion of the oxygen in the fluid is slowed, the corrosion potential at the austenitic stainless steel becomes anodic and local corrosion occurs easily. For instance, if a pump for conveying the seawater and its piping and valve system are made of austenitic stainless steel, crevice corrosion easily develops in the interface of two austenitic stainless steel components that are in contact with the seawater, such as the interface of the flanges attached to the suction port of the pump and the connecting pipe, the interface of the flanges attached to the discharge port of the pump and the connecting pipe, the mating surface of the casing parts, and the interface of the flanges for connecting the pipe to a valve. The mechanism of the development of crevice corrosion is as follows: the seawater entering the crevice that is unavoidably formed between two fitting parts is seldom replaced by the seawater outside the crevice, so the pH of the seawater within the crevice decreases and the concentration of chlorine ions in that seawater increases. As a result, a crevice corrosion develops due to the galvanic action that works between the interface and the surface other than the interface which is in contact with a substantially neutral environment, and such corrosion keeps going on unless the seawater within the crevice is replaced by the external seawater. One method that has been proposed to prevent such crevice corrosion is to fill the crevice with a joint sheet impregnated with an alkaline or oxidizing substance (Japanese Patent Public Disclosure Nos. 20954, 20955/1975). But such joint sheet can be used only in a crevice (i.e. the sheet has limited applicability) and its effectiveness does not last for an extended period.